Envelope tracking system for mimo

ABSTRACT

There is disclosed an amplifier arrangement comprising a plurality of amplifiers each arranged to amplify one of a plurality of different input signals, the arrangement comprising an envelope tracking modulator for generating a common power supply voltage for the power amplifiers, and further comprising an envelope selector adapted to receive a plurality of signals representing the envelopes of the plurality of input signals, and adapted to generate an output envelope signal representing the one of the plurality of envelopes having the highest level at a particular time instant as the input signal for the envelope tracking modulator.

Great Britain patent application number GB 1110732.3, filed on Jun. 24,2011, is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an amplification stage incorporatingmultiple power amplifiers having different input signals. The inventionis particularly but not exclusively concerned with multiple poweramplifiers in a transmitter of a MIMO system, for example in a cellularinfrastructure base station or micro base station of a mobilecommunication system.

BACKGROUND OF THE INVENTION

MIMO (multiple input, multiple output) systems are well known in thefield of mobile telecommunications. A MIMO transmitter generatesmultiple waveforms from a single signal source, which are amplified bymultiple power amplifiers to provide multiple signal paths. Thetransmitted signals on the multiple paths are detected at one or morereceivers.

It is well known in the art of RF power amplification to utiliseenvelope tracking modulated power supplies for power amplifiers in orderto improve various parameters of the power amplifier and in particularthe DC (direct current) to RF (radio frequency) power conversionefficiency.

The efficiency of a MIMO system can be improved by providing an envelopetracking modulated power supply for each individual power amplifier inthe MIMO transmitter. The envelope tracking power supplies for eachpower amplifier are modulated so that the power supply voltage tracksthe envelope of the input signal to be amplified. In a typical efficientarrangement one of a plurality of power supplies is selected independence on the input signal envelope, and then adjusted by an errorcorrection means to more closely track the envelope.

Whilst the use of an envelope tracking modulated power supply providesan efficient amplification arrangement, it increases the overheadassociated with the MIMO transmitter since an envelope trackingmodulator stage is required for each power amplifier of the MIMO system.

It is an aim of the present invention to provide an improved arrangementfor providing an envelope tracking modulated power supply inarrangements where multiple power amplifiers receive different inputsignals.

SUMMARY OF THE INVENTION

There is provided an amplifier arrangement comprising a plurality ofamplifiers each arranged to amplify one of a plurality of differentinput signals, the arrangement comprising an envelope tracking modulatorfor generating a common power supply voltage for the power amplifiers.

The amplifier arrangement may further comprise an envelope selectoradapted to receive a plurality of signals representing the envelopes ofthe plurality of input signals, and adapted to generate an outputenvelope signal representing the one of the plurality of envelopeshaving the highest level at a particular time instant as the inputsignal for the envelope tracking modulator. The envelope selector may bearranged to generate a composite envelope signal as the output envelopesignal. The composite envelope signal may transition between theenvelopes of ones of the plurality of different input signals independence upon the one of the plurality having the instantaneouslargest amplitude.

The amplifier arrangement may further comprise a plurality of envelopeprocessing stages each for receiving one of the plurality of inputsignals and for generating a signal representing the envelope of saidinput signal. Each of the envelope processing stages may include anon-linear mapping stage for mapping its input to its output. Each ofthe envelope processing stages may include a delay for delaying thesignal in the supply path relative to the signal in the input path tothe amplifier.

The amplifier arrangement may further comprise a moving average filterto remove discontinuities from the composite envelope signal.

The amplifier arrangement may further comprise a plurality ofpre-distortion stages, for receiving the respective plurality of inputsignals, and further arranged to receive said composite envelope signal,and provide a pre-distorted input signal to the input of the respectivepower amplifier.

The amplifier arrangement may further comprise a feedback receiverarranged to selectively receive the output of one of the plurality ofamplifiers and selectively provide an additional input to the one of theplurality of pre-distortion stages associated with the amplifier, toadaptively adjust the pre-distortion coefficients of the pre-distortionstage to minimise distortion at the output of the amplifier. Thefeedback receiver may be arranged to switch between all of the pluralityof amplifiers in a sequence.

The plurality of input signals may be derived from a common signalsource.

A multi-input, multi-output, MIMO, transmitter stage may include anamplifier arrangement as defined. The MIMO transmitter stage may be foruse in a cellular base station.

The invention may further provide a method of amplification in anamplifier arrangement comprising a plurality of amplifiers each arrangedto amplify one of a plurality of different input signals, the methodcomprising providing an envelope tracking modulator for generating acommon power supply voltage for the power amplifiers.

The method may further comprise receiving a plurality of signalsrepresenting the envelopes of the plurality of input signals, andgenerating an output envelope signal representing the one of theplurality of envelopes having the highest level at a particular timeinstant as the input signal for the envelope tracking modulator.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is now described by way of example with referenceto the accompanying Figures in which:

FIG. 1 illustrates an example of a prior art MIMO system;

FIG. 2 illustrates an example of a MIMO system incorporating anembodiment of the present invention;

FIG. 3 illustrates exemplary waveforms in an embodiment of theinvention;

FIG. 4 illustrates a modification to the arrangement of FIG. 2 in afurther embodiment; and

FIG. 5 illustrates further exemplary waveforms in an embodiment of theinvention.

DETAILED DESCRIPTION

The invention will now be described with further reference to theexemplary RF amplification architecture of a MIMO transmitter system ofa mobile communication system. Whilst the invention and its embodimentsmay be advantageously utilised in such an environment, the invention andits embodiments are not limited in their applicability to the exemplaryarchitectures and implementations as illustrated.

With reference to FIG. 1 there is illustrated an implementation ofconventional envelope tracking techniques in a MIMO transmitter.

The MIMO transmitter includes a digital signal processing blockincluding a signal source 114, a plurality of waveform processing blocks112 a to 112 d, a plurality of envelope processing blocks 108 a to 108d, and a plurality of digital pre-distortion blocks 110 a to 110 d; aplurality of envelope modulators 106 a to 106 d; a plurality of RFamplifiers 104 a to 104 d; and a plurality of antennas 102 a to 102 d.

The signal source 114 may generate a signal for transmission on awireless interface configured in accordance with an appropriatetelecommunications standard. The signal source 114 provides a basebandsignal which forms an input to each of the plurality of waveformprocessing blocks, denoted by reference numerals 112.

In the arrangement of FIG. 1 there is provided four waveform processingblocks denoted 112 a to 112 d. In general there may be provided anynumber of waveform processing blocks.

Each waveform processing block 112 a to 112 d provides a waveform at itsoutput which is a baseband signal representative of the RF waveformcarried by one channel of the MIMO transmitter, and is typically adigital representation of the baseband in-phase and quadrature (I and Q)signals of the RF carrier to be transmitted. This processing may includecrest factor reduction of the waveform.

The waveform at the output of each waveform processing block 112 a to112 d is provided at an input of respective envelope processing blocks108 a to 108 d and an input of respective DPD blocks 110 a to 110 d.Consistent with the provision of four waveform processing blocks thereare provided four envelope processing blocks and four DPD blocks. Analternative implementation is for the input to the envelope processingblock 108 to be taken from the output of DPD block 110.

Each envelope processing block determines the envelope of the respectivebaseband waveform generated by the respective waveform processing block,and may provide a non-linear mapping between said envelope and thedesired supply voltage. Such non-linear mapping provides a ‘shaped’envelope waveform. Without such mapping, the envelope waveform is‘unshaped’. The envelope processing block may also contain a variabledelay element to allow the timing between the supply path to therespective amplifier and the RF input path to that amplifier to bealigned.

Each DPD block 110 corrects for amplitude and phase distortion of itsassociated power amplifier. Typically the parameters of each DPD blockare updated by monitoring the associated power amplifier output andadjusting the parameters to minimise the distortion of the associatedpower amplifier output.

The outputs of the envelope processing blocks 108 and DPD processingblocks 110 form the outputs of the digital signal processing block 116.

There are provided a similar plurality of envelope modulators 106 andpower amplifiers 104. Thus in the described arrangement there areprovided four envelope modulators 106 a to 106 d and four poweramplifiers 104 a to 104 d. Again, in general, there may be provided anynumber of envelope modulators and power amplifiers.

It should be noted that FIG. 1 illustrates the main elements of theexemplary transmitter, but not all of the required elements. For exampleintermediate functional blocks may be needed between the DPD blocks andthe power amplifiers, such as digital-to-analogue converters, basebandto RF upconversion circuitry, power amplifier driver etc.

The outputs of the envelope processing blocks 108 provide inputs to therespective envelope tracking modulators 106, and the outputs of the DPDblocks 110 provide inputs to the respective power amplifiers 104.

The outputs of the respective envelope tracking modulators 106 provideinputs to the power supply terminals of the respective power amplifiers104. As known in the art, the envelope tracking modulators 106 a to 106d track the respective envelope signals to deliver an efficientmodulated power supply to the respective power amplifiers 104 a to 104d.

As can be seen in FIG. 1, a separate envelope tracking modulator isprovided for each of the multiple paths, such that there is a one-to-onearrangement between envelope modulators and power amplifiers. Eachindividual power amplifier 104 a to 104 d receives its own envelopemodulated power supply from its own envelope modulator 106 a to 106 d.

The output of each power amplifier is connected to a respective antenna102. Again, in the described example, there are four antennas 102 a to102 d associated with each of the respective power amplifiers 104 a to104 d.

The power amplifiers 104 feed the individual antennas 102, and in thereceiver there is provided multiple antennas for detecting thetransmitted signals. Such a MIMO implementation provides many more pathsbetween transmitters and receivers than a single antenna system.Different signals are amplified from the same source, and effectivelyconstitute independent waveforms.

With reference to FIG. 2 there is illustrated an embodiment of anenvelope tracking system for a MIMO architecture in accordance with theprinciples of the present invention. Where elements of FIG. 2 correspondto elements of FIG. 1 like reference numerals are used. The embodimentis described as an improvement to the exemplary arrangement of FIG. 1.

In accordance with the invention individual channels are still provided,with individual waveforms being processed in specific envelopeprocessing blocks 108 a to 108 d and DPD processing blocks 110 a to 110d. The plurality of power amplifiers 104 a to 104 d are provided, eachstill receiving the respective input signals for amplification from theoutputs of the DPD processing blocks 110 a to 110 d.

In accordance with the invention, the outputs of each of the envelopeprocessing blocks 108 a to 108 d are provided as inputs to an envelopeselection block 120 within the digital signal processing block 116. Theenvelope selection block 120 then provides an output to a singleenvelope tracking modulator 122. The single envelope tracking modulator122 then provides the modulated power supply signals for all of thepower amplifiers 104 a to 104 d.

The provision of the single envelope tracking modulator 122 to replacethe set of envelope tracking modulators of FIG. 1 introduces a potentialproblem, in that there is no longer a one-to-one correspondence betweenthe supply voltage to an individual power amplifier and the RF envelopeat the input of that power amplifier for any instant in time. As aconsequence it is no longer possible to determine the required DPDpre-correction with knowledge of only the instantaneous RF input power.

For each power amplifier 104 a to 104 b it is a requirement that thesupply voltage is higher than the minimum supply voltage which resultsin hard compression in the power amplifier in order for pre-distortionof the RF path to be possible.

The envelope selection block 120 is therefore arranged to determine themaximum envelope signal from the individual envelope signals providedfrom amongst the envelope processing blocks 108 a to 108 d. The envelopemodulator 122 then generates a modulated power supply signal at itsoutput in dependence upon the one of the multiple envelope signals whichis the highest at any one instant. The power supply to all of the poweramplifiers 104 a to 104 d at any instant in time is thus based on theone of the envelopes that is the highest.

The envelope selection block 120 and the envelope tracking modulator 122thereby ensure that all power amplifiers have a sufficiently high supplyvoltage to allow pre-distortion of the RF input path to correct for thesupply voltage induced distortion.

The operation of the envelope tracking modulator 122 can be furtherunderstood with reference to FIG. 3. FIG. 3 illustrates the plots of twoenvelope waveforms denoted by reference numerals 220 and 222, and awaveform 224 representing the composite signal formed by thosewaveforms. As can be seen at any instant the composite envelope 224comprises the one of the individual envelopes 220 and 222 which has thelargest amplitude.

The envelope selection block 120, as noted above, is adapted todetermine the highest envelope signal at any instant, and thus willoutput different envelope signals in accordance with its determinations.Short cusps may occur where the envelope selection block 120 jumps fromone waveform to another waveform. A moving average filter may thus beprovided at the output of the envelope processing block 120, asrepresented by block 121 in FIG. 2, in order to smooth the transitionpoints. Such a moving average filter also ensures that a higher voltagethan is needed is always provided as the supply voltage.

This is further illustrated with respect to FIG. 4. First and secondwaveforms 130 and 132 represent two envelopes, and waveform 134represents a composite of those envelopes. As can be seen by the regiondenoted by reference numeral 136, a short cusp appears as the compositeenvelope is determined by one envelope and then another, i.e. as thecomposite envelope ‘jumps’ from one envelope to another envelope.

Thus in accordance with the invention a single envelope trackingmodulator can be used to drive all power amplifiers.

A further problem arises because the instantaneous output power of apower amplifier is dependent on both its instantaneous input power andits instantaneous supply voltage. A conventional DPD only has knowledgeof the instantaneous input power and hence cannot systematically correctfor power amplifier output power errors which result from changes ininstantaneous supply voltage which are uncorrelated with theinstantaneous input power.

To address this problem, each of the DPD processing blocks 110 a to 110d is provided with information on the instantaneous supply voltage tothe power amplifiers as well as a digital representation of the basebandI and Q components of its respective RF carrier for its respectiveamplifier. The information of the instantaneous supply voltage isprovided from the output of filter block 121 as shown in FIG. 2. Withthe power supply voltage information and the baseband I and Qinformation the DPD can correct for distortion.

The specific DPD processing to correct for distortion based on theseinputs will be implementation dependent.

The DPD pre-correction applied to each power amplifier in the respectiveDPD blocks 110 may be fixed or adaptive. Where fixed pre-correction isused this may be predetermined by design, or adjusted at devicemanufacture.

Where adaptive pre-correction is used, a receiver may be used to samplethe power amplifier output signal thereby creating a feedback path foradaptation of the DPD parameters. As the required update rate of the DPDparameters is very low, the feedback receiver may be multiplexed betweenall transmit channels, as indicated in FIG. 5.

As shown in FIG. 5, the arrangement of FIG. 2 may be further modified insuch an adaptive arrangement to include a feedback receiver 140, anoutput switch 142 and an input switch 144. The output switch 142 iscontrolled to select one of the outputs of the power amplifiers 104 a to104 d to the feedback receiver 140, and the input switch 144 iscontrolled to connect the output of the feedback receiver 140 to anadditional input of one of each of the DPD blocks 110 a to 110 d.

The switches 142 and 144 are controlled by a common control signal suchthat when the output of one of the amplifiers 104 a to 104 d isselected, the input to the corresponding DPD block is also selected. Theswitches may be controlled to switch in fixed periods following apredetermined cycle.

In general, therefore, a single envelope tracking modulator is used forproviding a modulated supply voltage to multiple power amplifiers whereeach of the multiple power amplifiers has a different input signal. Ingeneral, the inventive techniques applies to any such arrangement whichdoes not provide in a one-to-one mapping between instantaneous supplyvoltage and instantaneous RF input envelope, not just a MIMOarrangement.

The invention is described herein with reference to particular examplesand embodiments, which are useful for understanding the invention andunderstanding a preferred implementation of the invention. The inventionis not, however, limited to the specifics of any given embodiment, norare the details of any embodiment mutually exclusive. The scope of theinvention is defined by the appended claims.

What is claimed is: 1.-14. (canceled)
 15. A pre-distortion stage for anamplifier arrangement comprising: an input for receiving one of aplurality of input signals; an input for receiving a composite envelopesignal, the composite envelope signal representing an envelope of theone of the plurality of input signals having the highest level at aparticular time instant; and an output for providing a pre-distortedinput signal to one of a plurality of power amplifiers.
 16. Thepre-distortion stage of claim 15 wherein each of the plurality ofamplifiers receives a common supply voltage in dependence on thecomposite envelope signal.
 17. A plurality of pre-distortion stagesaccording to claim 15, each for receiving one of the respectiveplurality of input signals, each for receiving the composite envelopesignal, and each for providing a pre-distorted input signal to the inputof a respective power amplifier.
 18. The pre-distortion stage of claim15 further comprising an input for selectively receiving an output froma feedback receiver, the feedback receiver arranged to selectivelyreceive the output of one of the plurality of power amplifiers toadaptively adjust the pre-distortion coefficients of the pre-distortionstage to minimise distortion at the output of the associated poweramplifier.
 19. The amplifier of claim 18 wherein the feedback receiveris arranged to switch between all of the plurality of amplifiers in asequence.
 20. The pre-distortion stage of claim 15 wherein the inputsignal is derived from a signal source common to inputs to otherpre-distortion stages.
 21. A multi-input, multi-output, MIMO,transmitter stage including a pre-distortion stage according to claim15.
 22. The MIMO transmitter stage of claim 21 for use in a cellularbase station.
 23. A method for pre-distorting in an amplifierarrangement comprising: receiving one of a plurality of input signals;receiving a composite envelope signal, the composite envelope signalrepresenting an envelope of the one of the plurality of input signalshaving the highest level at a particular time instant; and in dependenceon the received signals providing a pre-distorted signal to one of aplurality of power amplifiers.
 24. The method of claim 23 furthercomprising receiving a common supply voltage in dependence on thecomposite envelope signal at each of the plurality of amplifiers.
 25. Amethod according to claim 23, further comprising receiving one of therespective plurality of input signals at one of a plurality ofpre-distortion stages, receiving the composite envelope signal at eachof the plurality of pre-distortion stages, and providing a pre-distortedsignal to the input of a respective power amplifier from eachpre-distortion stage.
 26. The method of claim 23 further comprisingselectively receiving an output from a feedback receiver arranged toselectively receive the output of one of the plurality of poweramplifiers, and adaptively adjusting pre-distortion coefficients tominimise distortion at the output of the associated power amplifier. 27.The method of claim 26 wherein the feedback receiver is arranged toswitch between all of the plurality of amplifiers in a sequence.
 28. Themethod of claim 23 further comprising deriving the input signal from asignal source common to inputs to other pre-distortion stages.
 29. Themethod of claim 23 in a multi-input, multi-output, MIMO, transmitterstage.
 30. The method of claim 29 in a cellular base station.